Quad Flat No-Lead (QFN) packaging technology can be used to produce packaged die which are not significantly bigger than the actual die. QFN packages are fabricated by attaching the die to a metal leadframe and forming electrical connections between the leadframe and pads on the die using wirebonds. A moulding compound (which may also be referred to as a ‘mould’, ‘mold’ or ‘molding’ compound) is then used to encapsulate the die and the wirebonds and form the package. One face of the leadframe is exposed on the base of the QFN package and the exposed areas of metal may be used to attach the QFN package to a printed circuit board (PCB) or other substrate, e.g. using solder paste which may be stencil printed onto the PCB.
Chip-on-Lead (CoL) is a variant of QFN in which the die is mounted directly onto all or some of the leads (which may also be referred to as ‘terminals’ or ‘electrical terminals’), often in addition to mounting onto a separate die attach region of the leadframe, which may be referred to as the paddle. Use of CoL QFNs may enable smaller packages, because there is a reduction in the minimum distance between the edge of the die and the package edge (e.g. a reduction from around 1 mm to around 0.5 mm). FIG. 1 shows a comparison of a QFN package 101 and a CoL QFN package 102. Each example shows a cross-section through a portion of a package and shows the leadframe 111, 112, die 121, 122, adhesive 131, 132, wirebonds 141, 142 and moulding compound 151, 152.
As the areas of metallization on the underside of the package are of different sizes, problems may arise when soldering the QFN packages onto a substrate. In particular, during solder attachment where a large region of solder under the paddle, can cause the package to float up during the solder-reflow process, without a solder joint being formed to some of the smaller leads around the periphery of the device. The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known QFN packages and methods of manufacturing QFN packages.